Under the designation “premix burner” a burner device is known to persons skilled and active in the relevant art, with the feature that the fuel is burned only with a certain time interval after its intermixing with the air provided for the combustion.
During the operation of a conventional premix burner, when the feeding of fuel to the burner is increased, a state is often reached in which the combustion becomes unstable and acoustic vibrations are caused in the plant into which it is fitted. The acoustic vibrations are known by the term “combustion vibrations”. The combustion vibrations may be so large that they jeopardise the operation of the premix burner and the plant, of which the premix burner is an integral part. The tendency of a premix burner to form unstable combustion becomes all the greater, the more homogenous the mixture of fuel is and the air formed in the premix burner before the combustion. However, a mixture which is as homogeneous as possible is desired in view of the fact that the production of nitrous oxides during the combustion is lower, the more homogenous is the mixture. If the mixture is completely homogenous, the maximum temperature occurring during the combustion of the mixture assumes a minimum, and it is precisely this effect which is essential for an especially low production of nitrous oxides. European Patents EP 0 193 838 B1 and EP 0 589 520 B1 disclose such a device.
To stabilise the combustion of a premix burner, it has been proposed in U.S. Pat. Nos. 5,758,587 and 6,056,538 to Büchner et al to envelope the igniting mixture flowing from the burner with a veil of air and thus prevent vortices from forming in marginal regions of the mixture, in which vortices combustion processes take place, from which it may be assumed that they contribute substantially to the destabilisation of the combustion. This is achieved with the disadvantage of extracting air which is used to envelope the mixture from the actual combustion operation which markedly may increase the formation of nitrous oxides.
U.S. Pat. No. 6,152,724 which corresponds to European Patent 0 925 470 B1 describes a device for burning a fuel in air. This device includes a body having an axis and an annular passage formed therein for directing air in a meridional flow with regard to the axis; a swirl cascade connected to the body and imposing a swirl on the flow; an air flow delayer connected to the body and delaying a portion of the flow lying radially on an outside with regard to the axis relative to other portions of the flow; and a mixer connected to the body and intermixing fuel with the flow for forming a substantially homogeneous air/fuel mixture. The velocity in the flow, when the latter discharges from the device, is configured non-uniformly in the radial direction with regard to the axis. This is effected by the flow being locally disturbed in the annular passage by an appropriate obstacle in the form of a screen or the like, which is disposed at an appropriate point in the annular passage. But at the same time the homogeneity of the mixture of air and fuel in the flow is retained.
German Patent DE 198 39 085 C2 relates to a burner device for a firing installation, in particular a combustion turbine, with a main burner, which is a premixing burner, and a primary and secondary pilot burner. The primary pilot burner is surrounded by the main burner and centred with respect to the main axis of the main burner. The secondary pilot burner is placed at the outlet of the main burner, where the mixture of fuel and air enters the combustion chamber. The secondary pilot burner provides additional fuel through a number of orifices at the outlet of the main burner, which leads to a non-uniform contribution of fluid at the outlet of the main burner. To achieve this the installation of additional pipes for providing fluid a well as further mechanical features are necessary which makes the burner device more complicated and spacious.
International Patent Application WO 98/35186 A1 relates to an active method to suppress combustion vibrations in a combustion turbine plant. This method for active attenuation of a combustion oscillation in a combustion chamber uses at least two control elements, wherein an operating lever of the control elements requires the combustion oscillation to be measured only at a limited number of points. This is achieved mainly by using the symmetry of an acoustic vibration generated by self-excitation in the combustion chamber. All active methods to suppress and control combustion vibrations require additional electrical control equipment as well as sensors being exposed to the hot combustion gas.